Microelectronic devices having underfill materials with improved fluxing agents

ABSTRACT

An underfill material, such as a no flow underfill material, containing an anhydride adduct of a rosin compound is disclosed. In one aspect, the anhydride adduct of a rosin compound contains an organic rosin acid moiety and a substitute moiety for a hydroxyl group of a carboxylic acid attached at an acyl group of the organic rosin acid moiety. In another aspect, the anhydride adduct of the rosin compound contains a plurality of linked organic rosin acid moieties. Methods of using the underfill materials and packages formed by curing the underfill materials are also disclosed.

BACKGROUND

1. Field

An embodiment of the invention relates to a fluxing agent for anunderfill material.

2. Background Information

No flow underfill materials for microelectronic packages often containfluxing agents to help remove metal oxides from solder bumps duringsolder reflow. Commonly employed fluxing agents include low molecularweight carboxylic acids and anhydrides. A potential problem withemploying such low molecular weight agents is that voids may form due tovaporization of the fluxing agents during the elevated temperatures usedduring reflow or underfill material curing. The voids may concentratethermomechanical stresses, trap undesired moisture, or otherwise reducethe effectiveness of the underfill material and degrade devicereliability. Another potential problem, when employing carboxylic acids,is premature reaction with the underfill material, which may potentiallylead to increased viscosity, poor solder joint formation, and reduceproduction yields.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 shows an anhydride adduct of a rosin compound containing a singleorganic rosin acid moiety that may be used as a fluxing agent in anunderfill material, according to one embodiment of the invention.

FIG. 2 shows an anhydride adduct of a rosin compound containing aplurality of linked organic rosin acid moieties that may be used as afluxing agent in an underfill material, according to one embodiment ofthe invention.

FIG. 3 shows a method for introducing an anhydride adduct of a rosincompound into an underfill material, according to one embodiment of theinvention

FIG. 4 a shows a cross-sectional view of an applicator applying a noflow underfill material containing an anhydride adduct of a rosincompound over a substrate, according to one embodiment of the invention.

FIG. 4 b shows a cross-sectional view of a microelectronic device placedover the substrate and underfill material of FIG. 4 a, according to oneembodiment of the invention.

FIG. 4 c shows a cross-sectional view of a microelectronicdevice-substrate assembly formed by placing the solder bumps of themicroelectronic device into contact with the pads of the substrate,according to one embodiment of the invention.

FIG. 4 d shows a cross-sectional view of a flip chip microelectronicpackage after reflowing the solder bumps and curing the underfillmaterial, according to one embodiment of the invention.

FIG. 5 shows a cross sectional view of a portion of a microelectronicpackage containing a cured underfill material having a plurality oforganic rosin acid moieties (as indicated by asterisks) bonded thereto,according to one embodiment of the invention.

FIG. 6 shows an exemplary computer system in which an embodiment of theinvention may be implemented.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

I. Overview

Traditionally, the solder joints in flip chip devices have experiencedreliability problems due to thermo-mechanical stresses. There are oftendifferences in the coefficients of thermal expansion between themicroelectronic device, which often includes a silicon die, and thesubstrate, which often includes an FR-4 glass epoxy or similar printedcircuit board. The differences in the coefficients of thermal expansionmay lead to stresses and strains on the solder joints used to connectthe microelectronic device to the substrate during routine temperaturecycling excursions experienced in the. device. In certain cases, thestresses and strains may cause the solder joints to fail.

Underfill materials are generally introduced in the gap between themicroelectronic device and the substrate to help reduce thethermo-mechanical stresses and improve device reliability. The underfillmaterials, which often include filled epoxies, provide mechanicalstrength and are often stiff enough to absorb much of the stress andstrain due to the coefficient of thermal expansion mismatch. Theunderfill material generally also protects the active surface of themicroelectronic device from moisture and impurities. The use of suchunderfill materials, when properly selected and applied, maysignificantly reduce the stresses and strains on the solder joints. Itis not uncommon for the underfill material to help improve devicereliability by an order of magnitude, or more.

A no flow underfill process is a commonly employed process forintroducing the underfill material between the substrate and themicroelectronic device. In a representative no flow underfill process,an underfill material may be applied to the substrate before themicroelectronic device is assembled relative to the substrate.

No flow underfill materials often include a fluxing agent to removemetal oxides from solder bumps during the solder reflow. Traditionally,low molecular weight (MW) carboxylic acids, such as adipic acid (MW=146)or citric acid (MW=192), or low molecular weight anhydrides, such asmethyl hexahydro phthalic anhydride (MHHPA) (MW=160) or nadic methylanhydride (NMA) (MW=178), have been employed as fluxing agents.Generally, low molecular weight carboxylic acids and anhydrides have anappreciable vapor pressure at the elevated temperatures employed duringsolder reflow or underfill curing. These temperatures are often in arange between about 200° C. to 250° C. When heated to thesetemperatures, the fluxing agents may vaporize, and may potentially causevoids to form in the underfill material as it cures. Carboxylic acidsmay additionally potentially cause voiding as a result ofdecarboxylation or dehydration. The voids may concentratethermomechanical stresses, trap undesired moisture, or otherwise reducethe effectiveness of the underfill material and degrade devicereliability. Another potential problem, if a carboxylic acid fluxingagent is employed, is premature reaction with the underfill material.Such reaction may potentially alter the curing kinetics, prematurelyincrease the viscosity, and hinder solder joint formation.

II. Exemplary Anhydride Adducts Of Rosin Compounds

FIGS. 1-2 show exemplary anhydride adducts of rosin compounds. Anembodiment of the invention may include a method of introducing such acompound into an underfill material. Another embodiment of the inventionmay include a composition containing an underfill material and such acompound. Yet another embodiment of the invention may include a methodof applying a composition containing an underfill material and such acompound over a surface of a substrate and using the compound as afluxing agent. A still further embodiment of the invention may include amicroelectronic package containing an underfill material and a moiety ofsuch a compound bonded with the underfill material.

A. Compounds Containing Single Organic Rosin Acid Moieties

FIG. 1 shows an anhydride adduct of a rosin compound 100 containing asingle organic rosin acid moiety that may be used as a fluxing agent inan underfill material, according to one embodiment of the invention. Theanhydride adduct of the rosin compound includes a maleic anhydridemoiety 110, a single organic rosin acid moiety 120, and an ester moiety130. Before discussing the structure of the compound in greater detail,a brief discussion of rosins, and organic rosin acids, may bebeneficial.

Rosin has been used as a soldering flux for many years and is believedto possess good fluxing characteristics. Rosin generally refers to anaturally occurring organic material derived from trees or other plantsources. Exemplary rosins include, but are not limited to, wood rosin,gum rosin, tall oil rosin, oleoresin, other plant rosins. Such rosinsgenerally include a mixture of organic acids and a variety of othercompounds, such as fatty acids and triterpenes. The composition of therosin, and the relative proportions of the different organic acids,generally depends on the source of the rosin, such as the wood fromwhich it is derived, and upon other factors, such as the temperaturesand chemical treatments used to process the rosin.

Organic acids commonly found in rosins include diterpene resin acids.Two types of diterpene resin acids commonly found in rosins are abietictype acids (for example levopimaric acid, abietic acid, neoabietic acid,palustric acid, and dehydroabietic acid) and pimaric type acids (forexample pimaric, isopimaric, and sandaracopimaric). Some diterpene resinacids, such as abietic acid, are cyclic isoprenoid acids.

Referring again to FIG. 1, the illustrated organic rosin acid moiety isbased on an abietic type acid, such as levopimaric acid, although thisis not required. Abietic type acids generally include three fusedsix-membered rings, which are based on four isoprene units. Otherorganic rosin acid moieties that may optionally be employed include, butare not limited to, those derived from other organic rosin acids, resinacids, diterpene resin acids, cyclic isoprenoid acids, fused polycyclicisoprenoid monocarboxylic acids, abietic type acids, pimaric type acids,pimaric acid, isomers of pimaric acid, isopimaric acid, sandaracopimaricacid, and combinations of such acids. Either isolated acids orcombinations of acids may be employed. The terms organic rosin acid,resin acid, diterpene resin acid, cyclic isoprenoid acid, fusedpolycyclic isoprenoid monocarboxylic acid, abietic type acid (or pimarictype acid), abietic acid or isomers thereof (or pimaric acid or isomersthereof) are all related, and generally appear in order from moregeneral at the beginning of the list to more specific at the end. Asused herein, the term rosin compound does not impose any limitation thatthe compound be derived from a rosin, and includes synthetic compoundsthat are also found in rosins, as well as derivatives of compounds thatare found in rosins, synthetic or otherwise.

Referring again to FIG. 1, the maleic anhydride moiety is bonded to theorganic rosin acid moiety at a Diels-Alder addition site, where theorganic rosin acid moiety had an unsaturated double bond. Levopimaricacid includes a conjugated diene containing two double bonds that areseparated by one single bond. A dienophile anhydride, such as maleicanhydride, containing a double bond having one or two carbon, nitrogen,or oxygen atoms, may be reacted or adducted with the conjugated diene ofan organic rosin acid, to give a Diels-Alder adduct or anhydride adduct.It will be appreciated that other dienophile anhydrides, whether cyclicor non-cyclic, may optionally be employed, and that the addition sitemay potentially have other positions if other organic rosin acids areemployed.

The ester moiety 130 includes an acyl group (R′CO—) of the organic rosinacid moiety bonded to an —OR group, where the R′ includes the organicrosin acid moiety, and the R may include, but is not limited to, analkyl group, such as a methyl, ethyl, propyl, butyl, or other grouphaving, for example, between 1 to 10 or 1 to 20 carbons, or an arylgroup, such as a phenyl group (—C₆H₅). The ester moiety represents anesterification reaction product that may be formed by an esterificationreaction of the carboxylic acid group of the organic rosin acid and analcohol, phenol, or other compound providing the —OR group. The —ORgroup is a substitute group or moiety in place of an acidic hydroxyl(—OH) group of a carboxylic acid group that was natively present in theorganic rosin acid corresponding to the organic rosin acid moiety. Theuse of an ester or other replacement moiety for the acidic hydroxylgroup of the carboxylic acid is not required, but may assist ineliminating the carboxylic acid group, which may react with theunderfill material and alter the curing kinetics thereof, and which maypotentially decarboxylate or otherwise cause voiding. Accordingly,another potential advantage of the anhydride adduct of the rosincompound is reduced voiding and less alteration of the underfillmaterial compared to native organic rosin acids. Alternatively, inanother embodiment of the invention, the ester moiety may be omitted,and the acidic hydroxyl group of the organic rosin acids carboxylic acidgroup retained.

The fluxing agent shown in FIG. 1 combines the good fluxingcharacteristics traditionally associated with rosins and the epoxyhardening characteristics traditionally associated with anhydrides.Also, the ester moiety replaces the hydroxyl of the carboxylic acidgroup, which may prematurely react with underfill material, andpotentially hinder solder joint formation.

Moreover, the fluxing agent contains an organic rosin acid moiety havinga molecular weight of approximately 300, which is substantially higherthan the molecular weights of the low molecular weight carboxylic acidsand anhydrides commonly employed as fluxing agents. When combined withthe maleic anhydride moiety, which has a molecular weight of about 100,the fluxing agent achieves a molecular weight that is at least about400, plus any additional molecular weight provided by the ester moiety.It is typically the case that the vapor pressure of organic compoundsdecreases with increasing molecular weight. At this molecular weight,the vapor pressure of the fluxing agent at reflow or curingtemperatures, which are typically in a range between about 200° C. to250° C., are expected to be quite small. Accordingly, the use of suchhigh molecular weight fluxing agents may help to reduce voidingtraditionally caused by fluxing agent vaporization.

B. Compounds Containing A Plurality Of Linked Organic Rosin AcidMoieties

FIG. 2 shows an anhydride adduct of a rosin compound 200 containing aplurality of linked organic rosin acid moieties that may be used as afluxing agent in an underfill material, according to one embodiment ofthe invention. The anhydride adduct of the rosin compound includes afirst maleic anhydride moiety 210, a first organic rosin acid moiety220, a linking ester moiety 230, a second organic rosin acid moiety 240,and a second maleic anhydride moiety 250. The compound includes an esterof a first organic rosin acid moiety linked to an ester of a secondorganic rosin acid moiety. The linking ester moiety links the first andthe second organic rosin acid moieties and approximately doubles themolecular weight of the fluxing agent compared to the fluxing agentshown in FIG. 1.

The first maleic anhydride moiety 210 is bonded to the first organicrosin acid moiety 220 at a first Diels-Alder addition site, where thefirst organic rosin acid had an unsaturated double bond. Likewise, thesecond maleic anhydride moiety 250 is bonded to the second organic rosinacid moiety 240 at a second Diels-Alder addition site, where the secondorganic rosin acid had an unsaturated double bond. In the illustratedfluxing agent, both organic rosin acid moieties are based on levopimaricacid, although this is not required. In another embodiment, otherpotentially different organic rosin acid moieties may be employed. Otherpotentially different dienophile anhydrides may also optionally be used,and the addition sites may potentially be different if other organicrosin acids are employed.

The linking ester moiety 230 is bonded or esterified to the first rosinacid moiety and replaces the acidic hydroxyl group (—OH) of thecarboxylic acid that was natively present in the organic rosin acid fromwhich the first organic rosin acid moiety was derived. Likewise, theester moiety is bonded or esterified to the second rosin acid moiety andreplaces the acidic hydroxyl group of the carboxylic acid that wasnatively present in the organic rosin acid from which the second organicrosin acid moiety was derived. As discussed above, substituting thehydroxyl groups may help to avoid a reaction with the underfillmaterial, which could potentially increase the viscosity of theunderfill material, and hinder solder joint formation. This may alsopotential help to avoid a decarboxylation reaction, which could producegas, and cause voids.

The ester moiety joins or otherwise links, and dimerizes, the first andthe second organic rosin acid moieties. The ester moiety, in theillustrated embodiment, includes an aryl group, similar to a phenylgroup (—C₆H₅), but in which two hydrogens, in para positions relative toone another, have been removed. The corresponding ring carbons where thehydrogen atoms have been removed have been attached through the esterlinkages to the first and the second organic rosin acid moieties,respectively. One function of the ester moiety is to link a plurality oforganic rosin acid moieties and thereby significantly increase, ormultiply, the molecular weight of the fluxing agent.

The plurality of linked organic rosin acid moieties give the fluxingagent shown in FIG. 2 a molecular weight which is almost twice that ofthe fluxing agent shown in FIG. 1. The molecular weight of the fluxingagent is greater than 800 (actual MW is about 870). The vapor pressureof such a fluxing agent is expected to be less than approximately 5 mmHg(millimeters of mercury), at a temperature of about 250° C. Such a lowvapor pressure is generally considered to be negligible and the amountof voiding due to fluxing agent vaporization is also expected to benegligible. This reduction of voiding may help improve devicereliability and help extend operational life.

The particular illustrated ester moiety is not required, and other estermoieties may also optionally be employed. As one example, instead of ahydroquinone (aka 1,4-benzenediol) moiety, a catechol (aka1,2-benzenediol) or a resorcinol (aka 1,3-benzenediol moiety mayoptionally be employed. As another example, other diols or compoundscontaining two hydroxyl groups, such as butanediol or other alkanediols,or ethylene glycol, may optionally be employed. Triols or other polyolsmay potentially be employed and may potentially link three or moreorganic rosin acid moieties via three or more ester linkages. Exemplarypolyols include but are not limited to glycerol, trimethylol ethane,trimethylol propane, trimethylol butane, pentaerythritol, sorbitol, or acombination thereof. Also non-ester replacement moieties for thehydroxyl group of the carboxylic acid may also potentially be employed.For example, thiols and other replacement groups may potentially beused, if desired.

III. Introducing Anhydride Adduct of Rosin Into Underfill Material

FIG. 3 shows a method 300 for introducing an anhydride adduct of a rosincompound into an underfill material, such as a no flow underfillmaterial, according to one embodiment of the invention. The methodincludes providing an underfill material, at block 310, and providing ananhydride adduct of a rosin compound, at block 320. The materials may beprovided in any desired order.

Suitable underfill materials include but are not limited to epoxymaterials, cyanate ester materials, acrylic materials, and otherunderfill materials, and no flow underfill materials known in the arts.Three commonly employed epoxy underfill materials that are suitable areepoxy-anhydride underfill materials, epoxy-amine underfill materials,and epoxy-phenol underfill materials. These underfill materials arecommercially available from numerous sources. An exemplary no flowunderfill material includes Hysol FF2200, commercially available fromLoctite, of Rocky Hill, Conn. The underfill material may include avariety of optional components, such as fillers (for example silicafillers) to modify a coefficient of thermal expansion or other propertyof the underfill material, adhesion promoters, hardeners, or otheroptional chemical additives.

Referring again to FIG. 3, the anhydride adduct of the rosin compoundmay be introduced into the underfill material, at block 330. Theunderfill material may be added to a bowl, container, stirred tank,inline pipe mixer, or other material combination device. If desired, amaterial combination device with cooling capability may be employed tocool the materials. The materials may be cooled to a temperature that isless than room temperature and greater than a freezing point of theunderfill material. Such cooling, although not required, may beappropriate to help suppress curing reactions.

Then, the anhydride adduct of the rosin compound may be introduced oradded into the underfill material contained in the fluid combinationdevice. The anhydride adduct of the rosin compound may be added in anamount that is appropriate for such a fluxing agent. For example, theweight ratio of fluxing agent to underfill material (for a fluxing agentwith a weight similar to those shown in FIGS. 1 or 2) may be in a rangebetween about 1:1000 to 1:10, or about 1:500 to 1:15, or about 1:50 to1:25.

If desired, an alcohol, such as an alkanol or glycol, may be introducedinto the underfill material. The alcohol may be added in an amount thatis sufficient to help to transform an anhydride group into a carboxylicacid group (for example open the anhydride ring of a maleated rosin)during the elevated temperatures leading up to reflow or cure. By way ofexample, the weight of glycol or a similar alcohol to underfill materialmay be in a range between about 1:1000 to 1:20, or about 1:100 to 1:50.It is noted that inclusion of such an alcohol is not required and thesmall amount of moisture typically present in the underfill material maybe used to perform such transformation of the anhydride, although withperhaps less regularity and control.

If desired, fillers, or other optional conventional components, if theyare not already included in the provided underfill material, may beintroduced into the underfill material. The components may be introducedeither before, after, or during the introduction of the anhydride adductof the rosin compound.

If desired, the underfill material may then be mixed to help distributethe anhydride adduct of the rosin compound. After any mixing, theunderfill material may optionally be degassed under a vacuum to removeair bubbles trapped in the material during mixing. A low vacuum is oftensufficient. After any degassing, the underfill material may be used, orstored until needed. The underfill material is often cooled duringstorage to help suppress curing reactions and increase the storage life.The desired cooling temperature generally depends on the underfillmaterial, although a temperature around −40° C. may be appropriate foran epoxy resin.

In this embodiment of the invention, the anhydride adduct of the rosincompound has been introduced into a liquid underfill material, althoughthe invention is not so limited. In an alternate embodiment of theinvention, an anhydride adduct of a rosin compound may be included in asolid encapsulant material, such as an Anisotropic Conductive Adhesive(ACA), a resin sheet underfill, wafer-level solid film underfill, apowder that may be melted to form a film, or a paste containing a solidand a liquid. In a representative example, an anhydride adduct of arosin compound may be introduced into the solid or film before it issolidified, may be introduced around particles of a powder, or may beincluded in a liquid of a paste. Also, the use of a crosslinkingunderfill material is not required, and non-thermosetting underfillmaterials, such as thermoplastic underfill materials, may alsooptionally be employed. Such non-thermosetting underfill materials mayfacilitate reworkability.

IV. Fabricating Flip Chip Microelectronic Package Using UnderfillMaterial Containing Anhydride Adduct Of Rosin Compound

FIGS. 4 a-4 d show cross-sectional views representing different stagesof a method for fabricating a flip chip microelectronic package 450using a no flow underfill material containing an anhydride adduct of arosin compound, according to one embodiment of the invention.

FIG. 4 a shows a cross-sectional view of an applicator 405 applying a noflow underfill material 410 containing an anhydride adduct of a rosincompound over a substrate 415, according to one embodiment of theinvention. The underfill material may be prepared as described in FIG.3, or otherwise. The substrate, which may include a printed circuitboard, has a contact surface 417 having a plurality of pads 420 attachedthereto. The pads may include a metal or other conductive material tocarry an electrical signal. As used herein the term metal includesalloys and other metal-containing mixtures. The applicator, which mayinclude a conventional no flow underfill dispenser, applies the no flowunderfill material over the surface of the substrate and over thecontact pads. The underfill material may be applied over at least a flipchip bonding location of the substrate where a microelectronic device,such as a die, is to be attached. A sufficient quantity of the underfillmaterial should be applied so that voids do not occur as a result ofincomplete filling, but applying too much of the underfill materialshould be avoided, as this may lead to die floating and increased riskof forming an incomplete solder joint.

Although the name would seem to indicate otherwise, the no flowmaterials are generally liquids, and often do in fact flow duringapplication and assembly. As used herein, the term no flow underfillmaterial is used for its understood meaning in the art, and not to implythat the underfill materials do not flow. Other terms that may be usedrelatively interchangeably with no flow underfill material includepre-applied underfill materials, dispense first underfill materials, andapply before assemble underfill materials.

FIG. 4 b shows a cross-sectional view of a microelectronic device 425placed in position relative to, and in this case over, the substrate 415and underfill material 410 of FIG. 4 a, according to one embodiment ofthe invention. The applicator is removed after any application ordispensing of the underfill material. Then, the microelectronic device,which may include a semiconductor die, is brought over the substrate.The particular logic included in the microelectronic device is not alimitation of the invention. Examples of microelectronic devicesinclude, but are not limited to, processors (for examplemicroprocessors), ASICs (Application Specific Integrated Circuits), andhigh end DSPs (Digital Signal Processors). The microelectronic devicehas an active surface 427 with a plurality of solder bumps 430 attachedthereto. The solder bumps often include a metal, such as an alloy.Suitable metals for the solder bump include but are not limited totin-lead solders, and such lead-free solders as tin-silver solders,tin-copper solders, tin-silver-copper solders, and tin-bismuth solders.The microelectronic device is brought into close proximity to thesubstrate, with the active surface of the microelectronic device facingthe contact surface of the substrate, and with the plurality of solderbumps aligned over the plurality of pads.

FIG. 4 c shows a cross-sectional view of a microelectronicdevice-substrate assembly formed by placing the solder bumps 430 of themicroelectronic device 425 into contact with the pads 420 of thesubstrate 415, according to one embodiment of the invention. Afterplacing the microelectronic device over the substrate, as shown in FIG.4 b, the microelectronic device and the substrate may be broughttogether. The solder bumps may contact the underfill material, and mayexude the underfill material disposed between them and the pads, as theyare brought closer to the pads. If desired, a small compression forcemay be applied to help bring the solder bumps into contact with thepads. Die pick and place equipment is often employed to assemble themicroelectronic device relative to the substrate. The underfill materialis disposed around the pads and the solder bumps, between the substrateand the microelectronic device. In the illustrated embodiment of theinvention, the underfill material wets the right-hand and left-handedges of the microelectronic device, although this is not required.Other degrees of wetting, including no edge wetting, may also optionallybe employed.

The assembly shown in FIG. 4 c may be heated to reflow the solder bumpsand cure the underfill material. The solder bumps and the underfillmaterial may be heated from a starting temperature near ambient to amelting point temperature of a material of the solder bumps. Prior toreflow, the anhydride groups of the anhydride adduct of the rosincompound may hydrolyze with water, or react with an alcohol in theunderfill material, to form carboxylic acid groups. The carboxylic acidgroups are generally considered to be good fluxing agents and may assistwith cleaning the solder to help improve the integrity of the solderjoint, by removing metal oxides from the solder bumps and other metalsof the interconnect structure, prior to and during the reflowing of thesolder bumps. Depending upon the particular carboxylic acid groups, themetal oxides, and other factors, such cleaning may begin to occur at atemperature in a range between about 150° C. to 200° C.

The temperature of the underfill material and solder bumps may befurther increased to the melting point temperature of the solder. Themelting point temperature for eutectic tin-lead solders may be betweenabout 180° C. to 185° C., whereas the melting point temperature for atin-silver solder may be between about 220° C. to 225° C. Generally asthe melting point temperature of the solder is achieved, the solderbumps may begin to melt and reflow. During reflow, the solder bumps maybe formed into improved contact with the pads of the substrate. The padsand the reflowed solder bumps may form an interconnect structure thatcouples the microelectronic device with the substrate. The interconnectstructure may carry electrical signals and serve as a signaling path orsignaling medium for the package. It should be noted that theillustrated interconnect structure is not required, and otherinterconnect structures known in the arts may also optionally beemployed.

The carboxylic acid forms of the anhydride adducts of the rosincompounds may continue to remove metal oxides from the solder bumps,pads, and other portions of the interconnect structure, during themelting and reflow. Such removal of the metal oxides may improve solderjoint integrity and reliability. The compounds may also coat theinterconnect structures to help suppress oxidation during the highertemperatures typically employed during curing.

Generally, it may be appropriate for the no flow underfill material notto cure significantly prior to the solder reflow. Up to and during thesolder reflow, the anhydride adduct of the rosin compound generally doesnot react appreciably with the underfill material. A potential advantageis that the anhydride adduct of the rosin is not consumed and may serveits intended purpose as a fluxing agent. Another potential advantage isthat the viscosity does not appreciably increase due to prematurecuring. In this way, the solder joints may form without hinder from aviscous solution that could potentially prevent a solder joint fromforming.

The underfill material may be cured after the melting and the reflow ofthe solder bumps. Generally, the temperature of the underfill materialis increased above the melting point temperature of the solder materialto promote more rapid curing. For example, the temperature may beincreased between about 10° C. to 40° C., or about 20° C. to 30° C.higher than the melting point temperature of the solder material,depending upon the particular solder and underfill materials. Theanhydride adduct of the rosin compound may react with the underfillmaterial. For example, the anhydride group of the compound may reactwith an epoxy underfill material as typical in an anhydride-epoxy resinsystem.

FIG. 4 d shows a cross-sectional view of a flip chip microelectronicpackage 450 after reflowing the solder bumps and curing the underfillmaterial, according to one embodiment of the invention. Themicroelectronic package includes the microelectronic device 425, thesubstrate 415, the pads 420, reflowed solder bumps 430D, and a curedunderfill material 410D between the microelectronic device and thesubstrate, around the pads and the reflowed solder bumps. The curedunderfill material physically connects the microelectronic device to thesubstrate and may provide stiffness to help protect the interconnectstructure, the microelectronic device, and the substrate from damage dueto thermo-mechanical stresses. The cured underfill material may alsoprotect the surfaces of the interconnect structure, the microelectronicdevice, and the substrate from moisture and other causes of corrosion.

Due in part to the use of a high molecular weight adduct of a rosincompound as a fluxing agent, instead of a low molecular weightcarboxylic acid or anhydride, the cured underfill material may have lessvoids to concentrate stresses, trap moisture, or otherwise degradedevice reliability. As a result, the manufacturing yields and theoperational life of the microelectronic package may be improved.

FIG. 5 shows a cross sectional view of a portion of a microelectronicpackage containing a cured underfill material having a plurality oforganic rosin acid moieties (as indicated by asterisks) bonded thereto,according to one embodiment of the invention. The organic rosin acidmoieties may be derived from anhydride adduct of rosin compound fluxingagents. In one aspect, the organic rosin acid moiety may include anesterified organic rosin acid moiety having a substitute moiety in placeof an acidic hydroxyl group of a carboxylic acid group. In anotheraspect, the organic rosin acid moiety may be linked with a secondorganic rosin acid moiety. In yet another aspect, a linking moiety maybe esterified to the organic rosin acid moiety and a second organicrosin acid moiety may be esterified to the linking moiety. Such rosinmoieties in cured underfill materials may potentially be detected byinfrared spectroscopy, or other analysis methods.

V. Exemplary Computer Architecture Employing Microelectronic Package

Microelectronic packages such as those described herein may be used invarious electrical systems known in the arts. A computer system 600representing an exemplary laptop, desktop, workstation, host, or serverin which features of the present invention may be implemented will nowbe described with reference to FIG. 6.

The computer system contains a bus 601 to communicate information, and aprocessor 602 coupled with the bus 601 to process information. In oneembodiment of the invention, the processor may be packaged using a noflow underfill material having an anhydride adduct of a rosin compoundas a fluxing agent, as discussed elsewhere herein. The computer system600 further comprises a random access memory (RAM) or other dynamicstorage device 604 (referred to as main memory), coupled with the bus601 to store information and instructions to be executed by theprocessor 602. The main memory 604 also may be used to store temporaryvariables or other intermediate information during execution ofinstructions by the processor 602. Different types of memories that areemployed in some, but not all, computer systems include DRAM memories,SRAM memories, and Flash memories. The computer system 600 alsocomprises a read only memory (ROM) and other static storage devices 606coupled with the bus 601 to store static information and instructionsfor the processor 602, such as the BIOS. A mass storage device 607 suchas a magnetic disk, zip, or optical disc and its corresponding drive mayalso be coupled with the computer system 600 to store information andinstructions.

The computer system 600 may also be coupled via the bus 601 to a displaydevice 621, such as a cathode ray tube (CRT) or liquid crystal display(LCD), to display information to an end user. Typically, a data inputdevice 622, such as a keyboard or other alphanumeric input deviceincluding alphanumeric and other keys, may be coupled with the bus 601to communicate information and command selections to the processor 602.Another type of user input device is a cursor control device 623, suchas a mouse, a trackball, or cursor direction keys, to communicatedirection information and command selections to the processor 602 and tocontrol cursor movement on the display 621. Other devices 625 may becoupled with the bus 610 in some but not all computer systems. Exemplarydevices include but are not limited to a network interface, acommunication interface, an audio device, and a video input device.

VI. Preparing Anhydride Adducts of Rosin Compounds

Various preparations of maleated rosin esters are discussed in thepatent literature. U.S. Pat. No. 4,643,848 to Thomas et al., discussesmaking modified rosin polyhydric alcohol esters by reacting a rosin withan unsaturated dibasic acid, such as maleic anhydride, usingpentaerythritol (in Example 1 and 3-6) or sorbitol (Example 2). U.S.Pat. No. 6,583,263 to Gaudl (hereinafter referred to as the '263 patent)discusses the preparation of a maleated rosin ester in Example 1.Related approaches may be employed to prepare anhydride adducts of rosincompounds, such as the compounds shown in FIGS. 1 and 2.

For example, based on Example 1 of the '263 patent, about a mole ofrosin or an organic rosin acid, such as levopimaric acid or abieticacid, may be stirred under nitrogen for about 90 minutes at atemperature of about 180 C. Then, about a mole of maleic anhydride maybe added. The temperature may be maintained at about 180 C for about 15minutes, then the temperature may be raised to about 215 C and thecombination may be stirred at that temperature for about 1 hour.

Then, in order to make a compound having a single organic rosin acidmoiety, about a mole of an alcohol having a single hydroxyl group, suchas methanol, ethanol, propanol, or another alkanol having, for example,between 1 to 10 or 1 to 20 carbons, or an aryl group, such as a phenylgroup (—C6H5) may be added to the organic rosin acid. Alternatively, inorder to make a compound having two linked organic rosin acid moieties,about a half mole of a diol, such as butanediol, hydroquinone, orethylene glycol, may be added to the organic rosin acid.

If desired, magnesium oxide, or other esterification catalysts known inthe arts, such as phosphininc acid, may optionally be included, althoughthis is not required. If a catalyst is not desired, a longer time forreaction may be employed. After adding the alcohol, and any desiredcatalyst, the temperature may be raised to about 260 C to 270 C and thereaction mixture may be stirred until the acid value decreases to about30 mg KOH/g, or less. It is understood that the above example is to beconstrued as merely illustrative and that other preparation methodsknown in the arts may also optionally be employed.

VII. General Matters

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. Some of the methods are described in their most basic form, butoperations can be added to or deleted from any of the methods. In otherinstances, well-known circuits, structures, devices, and techniques havebeen shown in block diagram form or without detail in order not toobscure the understanding of this description. The description is thusto be regarded as illustrative instead of limiting.

It should also be appreciated that reference throughout thisspecification to “one embodiment” or “an embodiment” means that aparticular feature may be included in the practice of the invention.Similarly, it should be appreciated that in the foregoing description ofexemplary embodiments of the invention, various features are sometimesgrouped together in a single embodiment, Figure, or description thereoffor the purpose of streamlining the disclosure and aiding in theunderstanding of one or more of the various inventive aspects. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed invention requires more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive aspects lie in less than all features of a singleforegoing disclosed embodiment. Thus, the claims following the DetailedDescription are hereby expressly incorporated into this DetailedDescription, with each claim standing on its own as a separateembodiment of this invention.

In the claims, any element that does not explicitly state “means for”performing a specified function, or “step for” performing a specifiedfunction, is not to be interpreted as a “means” or “step” clause asspecified in 35 U.S.C. Section 112, Paragraph 6.

1-29. (canceled)
 30. A method comprising: applying an underfill materialcontaining an anhydride adduct of a rosin compound as a fluxing agentover a contact pad over a surface of a substrate; placing amicroelectronic device having an active surface, and a plurality ofsolder bumps disposed on a plurality of contact pads on the activesurface, relative to the substrate, with the plurality of solder bumpsdisposed within the no flow underfill material; removing a metal oxidefrom the solder bumps with the fluxing agent; reflowing the solder bumpsby heating to a temperature that is greater than a melting pointtemperature of the solder; and curing the underfill material.
 31. Themethod of claim 30, wherein said applying comprises applying anunderfill material containing an anhydride adduct of a rosin compoundincluding an ester of an organic rosin acid moiety over the surface ofthe substrate.
 32. The method of claim 30, wherein said applyingcomprises applying an underfill material containing an anhydride adductof a rosin compound including a plurality of linked organic rosin acidmoieties over the surface of the substrate.
 33. The method of claim 30,wherein said applying comprises applying an underfill materialcontaining an anhydride adduct of a rosin compound including an ester ofa first organic rosin acid moiety linked to an ester of a second organicrosin acid moiety over the surface of the substrate.
 34. The method ofclaim 30, wherein said applying comprises applying an underfill materialcontaining an anhydride adduct of a rosin compound including a firstorganic rosin acid moiety esterified to a linking moiety and a secondorganic rosin acid moiety esterified to the linking moiety over thesurface of the substrate.
 35. The method of claim 30, wherein saidremoving the metal oxide from the solder bumps comprises: opening a ringof a maleic anhydride moiety of the anhydride adduct of the rosincompound to form a carboxylic acid group by reacting the maleicanhydride moiety with a compound selected from the group consisting ofwater and an alcohol; and removing the metal oxide from the solder bumpwith the carboxylic acid group.
 36. The method of claim 30, furthercomprising bonding the anhydride adduct of the rosin compound to theunderfill material during said curing.
 37. A microelectronic packagecomprising: a microelectronic device; a substrate; an interconnectstructure including a solder material coupling the microelectronicdevice with the substrate; an underfill material around the interconnectstructure between the microelectronic device and the substrate; and anorganic rosin acid moiety in the underfill material, the organic rosinacid moiety derived from an anhydride adduct of a rosin compound fluxingagent.
 38. The microelectronic package of claim 37, wherein the organicrosin acid moiety comprises an esterified organic rosin acid moietyhaving a substitute moiety in place of an acidic hydroxyl group of acarboxylic acid group of an organic rosin acid corresponding to theorganic rosin acid moiety.
 39. The microelectronic package of claim 38,wherein the substrate comprises a printed circuit board, and wherein themicroelectronic device comprises a processor.
 40. The microelectronicpackage of claim 37, further comprising a second organic rosin acidmoiety linked with the organic rosin acid moiety.
 41. Themicroelectronic package of claim 40, wherein the substrate comprises aprinted circuit board, and wherein the microelectronic device comprisesa processor.
 42. The microelectronic package of claim 37, furthercomprising a linking moiety esterified to the organic rosin acid moietyand a second organic rosin acid moiety esterified to the linking moiety.43. The microelectronic package of claim 42, wherein the substratecomprises a printed circuit board, and wherein the microelectronicdevice comprises a processor.
 44. The microelectronic package of claim37, wherein the substrate comprises a printed circuit board, and whereinthe microelectronic device comprises a processor.
 45. Themicroelectronic package of claim 37, wherein the underfill materialcomprises a no-flow underfill material.
 46. The microelectronic packageof claim 37 included in a computer system having a DRAM.